In the public switched telephone network (PSTN) it is known to provide power from the exchange, or from an intermediate point such as a street side cabinet, to a remote point such as customer premises equipment or another intermediate point such as a drop-point, using the telephone wires themselves—the traditional twisted wire pair, typically made of copper wire. In the early development of telephone systems, when mains electricity was still comparatively rare, this allowed provision of service to premises which had no reliable power supply, or a supply which was unreliable, and it is still a useful feature of the PSTN system that communication is possible during a local power failure—in particular so that a call can be made to the electricity suppliers to alert them to the power failure.
There is an increasing demand for electronic communications equipment (e.g. wireless access points, CCTV cameras, machine-to-machine sensors, signage, and monitoring services such as for fire or burglar alarms). It is often necessary to install such equipments in locations where there is no existing power supply. The provision of mains electrical power in new locations can be costly, and there is therefore increasing interest in the capability to provide the electrical power the equipment requires over the same physical connection as that by which the electronic communications equipment transmits and receives data. This enables such services to be provided at locations which have no power supply (mains, generator, or battery). Provision may also be made to use the telecommunications connection to deliver a backup electricity supply for use in the event of failure of the usual power supply.
However, there are constraints regarding the amount of power that can be provided in this way over a wire pair, primarily due to the limitations of the electrical path that a twisted wire pair provides. In particular, the power supply must not interfere with the primary function of the path, to support communications. Moreover, there is a risk of the conductors overheating, or of insulation breakdown, if an attempt is made to carry large continuous currents for which they were not designed. The maximum power for which such connections were originally intended is sufficient to ring a telephone bell, but this is only required for a short period. It is not generally possible to use the power available over a standard telephone connection to supply more complex telecommunications equipment such as facsimile machines, answering machines, and computers.
The inventors have recognised that, for most of the time, any given wire in the network is carrying much less power than its maximum load, and that this spare capacity can be used to deliver power to devices in the network.
One way to increase the amount of available power is to use multiple pairs so that each pair carries current and voltage within safety limits, but the power can be aggregated at the remote device. The present invention, and the inventions which are the subject of the present applicant's related patent applications filed on the same date as the present application claiming priority from European patent applications 12250064.8 and 12250067.1, and entitled “Control of Line Power” and “Power Distribution for Telecommunications System”, all relate to managing the use of multiple wire lines to provide power to remote equipment. By arranging for the power required by an individual node to be distributed over several wire pairs, the limitations on any individual connection can be overcome.
However, the demand for connectivity makes it impracticable to identify sufficient spare (unused) wire pairs which can be dedicated simply for line powering. It is therefore desirable to use active pairs for line powering. On a telephone network, typically an active pair carries a dc component (−48 v), “POTS” (standard analogue telephony), and digital signalling using one of the DSL (digital subscriber line) systems such as HDSL, ADSL, VDSL, (respectively “high rate”, “asymmetric”, and “very high speed” DSL). To use an active pair to carry an injected power supply, the POTS is encapsulated into a G.711 64 kbps bitstream, and an increased DC bias voltage is added onto the pair, to provide the injected power supply. Essentially POTS and xDSL data signals are carried as modulations on the DC power supply. At the line termination, the xDSL, G.711 and DC are separated with a filter. A DC-DC power converter generates −48V, and the G.711 is converted back to POTS. A combiner takes the 48V, xDSL and POTS to recreate a standard telephony signal. This enables underutilised wiring to be used efficiently, and also enables the power supplied to each wire pair to remain within safe limits.
A power line injector has been developed which is capable of adapting its power output over a each of a plurality of wire pairs to provide a required current (˜60 milliAmps per pair) to a remote equipment. Examples of remotely powered equipment currently contemplated include a specially adapted wireless access point or other type of end terminal, such as a Power-over-Ethernet adaptor. This allows provision of services such as wireless communications; and the provision of electrical power for distribution over Ethernet communications cable networks even at locations where no other form of power is available. The power injector dynamically determines the amount of electrical power to inject into each individual wire pair so as to ensure that the maximum power that can be delivered safely is provided. In addition, providing power in this way enables power monitoring and related services to be provided by the telecommunications service provider.
To maximise the delivered power on each pair, the voltage and current on each wire pair should be as close as practicable to the maximum for which the wire pair is rated. However, when an alternating current signal, for example a DSL (digital subscriber loop) signal is also present on a wire pair, this will add a modulation onto the dc power supply, and the peak voltage may exceed the specified limit. For example, if there is a 200V limit, and the DSL signal has a peak voltage of 30V, then the maximum safe voltage should be limited to 170V. In practice a small tolerance would be allowed for manufacturing variance, temperature fluctuations, etc.
In some circumstances (for example in active street cabinets) where DSL is injected onto the network, the voltage of the alternating current signal has to be reduced in order to reduce cross-talk between two wire pairs. This is a particular problem where a street cabinet is handling wire pairs originating at nodes at different distances. For example, if a cable carries both ADSL from a telephone exchange and VDSL from an active cabinet some distance from the exchange, then if the VDSL were introduced at the same peak voltage as the ADSL, interference from the VDSL pair into the ADSL pair would be more significant than the anticipated interference from an ADSL pair into another ADSL pair. Typically cabinets are assigned a “CAL” value which reduces the voltage of the xDSL signal.
In other circumstances, some pairs may carry no xDSL, and be used solely for line powering.
To remain within allowable limits, the maximum dc voltage that can be carried on a given wire pair therefore depends on the magnitude of any ac signal it may be expected to carry. It would be possible to control power injection such that all wire pairs carry the same dc voltage, but this would be unduly restrictive because it would have to be set for the “worst case” (highest ac voltage) situation and would be significantly lower than that which the majority of lines could actually carry.
One way to allow the maximum possible voltage would be to individually set the line voltage on each pair. However, if the injector is controlled manually there would be significant risk of a misconfiguration, resulting in excessive voltages being present on the line or, less problematically, power being restricted unnecessarily. It would also require continuous monitoring to ensure timely responses to any changes in line usage.
The present invention provides a power injection system for delivering electrical power to one or more communications connections in a network, comprising a power control system for controlling the power to be delivered to the or each connection by the power injection system, the control system having a monitor for measuring the amplitude of signals present on the line, and an injector arranged to be controlled by the monitor such that it delivers a line voltage, such that the combined line voltage and signal amplitude complies with a predetermined limit for the line.
A further aspect provides a method for delivering electrical power to one or more communications connections in a network, wherein a power control system controls the power to be delivered to the or each connection, the control system being controlled in response to a sensor which measures the amplitude of signals present on the line and controls the injector such that it delivers a line voltage such that the combined line voltage and signal amplitude complies with a predetermined limit for the line.
The monitoring and power injection control system need not be co-located with the power injection point, and may control the delivery of power to connections at a plurality of locations in the network.